U.S. patent number 5,915,205 [Application Number 08/782,162] was granted by the patent office on 1999-06-22 for ingress noise cancellation for upstream signals on a cable television system using an antenna to determine local noise.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Walter Y. Chen.
United States Patent |
5,915,205 |
Chen |
June 22, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
Ingress noise cancellation for upstream signals on a cable
television system using an antenna to determine local noise
Abstract
A cable television system employs noise cancellation in order to
reduce radio frequency noise for upstream signals. The noise
cancellation can be located at the headend or located at the
juncture between the cable distribution system and the subscriber
distribution system. The system employing noise cancellation at the
headend has an antenna at a central distribution point on the cable
system, such as a fiber node. The system employing noise
cancellation at the juncture between the cable distribution system
and the subscriber distribution system employs a local antenna. The
noise received by the antenna is correlated with noise on the line
in an adaptive filter for cancelling or reducing the noise on the
line.
Inventors: |
Chen; Walter Y. (Plano,
TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
25125179 |
Appl.
No.: |
08/782,162 |
Filed: |
January 13, 1997 |
Current U.S.
Class: |
725/125; 725/127;
348/E7.07; 348/E7.052 |
Current CPC
Class: |
H04N
7/17309 (20130101); H04N 7/102 (20130101) |
Current International
Class: |
H04N
7/173 (20060101); H04N 7/10 (20060101); H04N
007/173 () |
Field of
Search: |
;348/6-13,192,193
;375/346-351 ;381/94.1,94.7
;455/3.1,3.3,4.1,4.2,5.1,6.1,6.2,6.3,501,507,511,290,63,67.1,67.2,67.4 |
Foreign Patent Documents
Primary Examiner: Flynn; Nathan
Attorney, Agent or Firm: Franz; Warren L. Kempler; William
B. Donaldson; Richard L.
Claims
What is claimed is:
1. A cable television system having noise interference cancellation
apparatus for upstream signals comprising:
a cable distribution system for distributing television signals
downstream from a central distribution point to a plurality of
subscribers and for transmitting data signals upstream from at
least one of said subscribers to said central distribution
point;
a signal controller located at a subscriber for transmitting said
upstream data signals to said central distribution point;
an antenna;
an adaptive filter coupled to said cable distribution system
between said signal controller and said central distribution point
and coupled to said antenna for cancelling radio frequency
interference in said upstream signal; said adaptive filter
comprising:
a first bandpass filter coupled to said antenna;
a first demodulator coupled to said first bandpass filter;
a first A/D converter coupled to said first demodulator;
a second bandpass filter coupled to said subscriber line;
a second demodulator coupled to said second bandpass filter;
a second A/D converter coupled to said second demodulator; and
a correlator coupled to said first and second A/D converters for
reducing interference in said subscriber line.
2. The apparatus of claim 1 further comprising a subscriber
distribution system located within the premises of the subscriber
and connected to said cable distribution system, wherein said
adaptive filter is located at a juncture of said cable distribution
system and said subscriber distribution system.
3. The apparatus of claim 2 wherein said antenna is local to said
subscriber.
4. The apparatus of claim 1 further comprising a D/A converter
coupled between said adaptive filter and said cable distribution
system.
5. The apparatus of claim 4 further comprising a modulator coupled
between said D/A converter and said cable distribution system.
6. The apparatus of claim 1 wherein said central distribution point
is an optical fiber node.
7. The apparatus of claim 1 further comprising a headend for
transmitting television signals to said central distribution point,
wherein said central distribution point is an optical fiber node
connected to said headend by an optical fiber, said adaptive filter
being located at said headend, said antenna being located at said
fiber node and transmitting antenna signals to said adaptive filter
via said optical fiber.
8. The apparatus of claim 1 wherein said adaptive filter is a FIR
filter.
9. A method of noise interference cancellation for upstream signals
in a cable television system comprising a cable distribution system
for distributing downstream television signals downstream from a
central distribution point to a plurality of subscribers and for
transmitting data signals upstream from at least on of said
subscribers to said central distribution point, the method
comprising:
receiving radio frequency signals which will interfere with said
upstream signals via an antenna to generate an antenna signal;
cancelling said radio frequency interference signals in an adaptive
filter coupled to said cable distribution system; wherein said
cancelling step comprises:
bandpass filtering said antenna signal to generate a filtered
antenna signal;
demodulating said filtered antenna signal to generate a first
demodulated signal;
converting said first demodulated signal to a first digital
signal;
bandpass filtering said upstream signals to generate a filtered
upstream signal;
demodulating said filtered upstream signal to generate a second
demodulated signal;
converting said second demodulated signal to a second digital
signal;
correlating said first and second digital signals for reducing
interference in said upstream signals.
10. The method of claim 9 wherein said cable television system
further comprises a subscriber distribution system located within
the premises of the subscriber and being connected to said cable
distribution system, the method further comprising the step of
locating said adaptive filter at a juncture of said cable
distribution system and said subscriber distribution system.
11. The method of claim 10 further comprising the step of locating
said antenna local to said subscriber.
12. The method of claim 9 further comprising the step of D/A
converting said correlated signal for generating a third digital
signal.
13. The method of claim 12 further comprising modulating said third
digital signal.
14. The method of claim 9 further comprising the step of locating
said central distribution point at an optical fiber node.
15. The method of claim 9 wherein cancelling step comprises
cancelling said radio frequency interference signals in a FIR
filter.
16. The method of claim 9 wherein said cable television system
further comprises a headend for transmitting television signals to
said central distribution point and said central distribution point
is an optical fiber node connected to said headend by an optical
fiber, the method further comprising the steps of:
locating said adaptive filter at said headend;
locating said antenna at said fiber node;
transmitting said antenna signals from said antenna to said
adaptive filter via said optical fiber.
17. The method of claim 16 further comprising the step of D/A
converting said correlated signal.
18. In a cable television system having a cable distribution system
for distributing television signals downstream from a central
distribution point to a plurality of subscribers and for
transmitting data signals upstream from at least one of said
subscribers to said central distribution point, noise interference
cancellation apparatus comprising:
an antenna for receiving interference signals and for generating an
antenna signal;
noise cancelling means for receiving said antenna signal and said
upstream signal for cancelling interference in said upstream signal
by correlating said antenna signal and said upstream signal; said
noise cancelling means comprising an adaptive filter including:
a first bandpass filter coupled to said antenna;
a first demodulator coupled to said first bandpass filter;
a first A/D converter coupled to said first demodulator;
a second bandpass filter coupled to said upstream signals;
a second demodulator coupled to said second bandpass filter;
a second A/D converter coupled to said second demodulator; and
a correlator coupled to said first and second A/D converters for
reducing interference in said upstream signals.
19. The apparatus of claim 18 wherein said antenna is local to said
noise cancelling means.
20. The apparatus of claim 18 wherein said interference is radio
frequency signals.
21. The apparatus of claim 20 wherein said radio frequency signals
are in the range of substantially 5-50 MHz.
22. The apparatus of claim 18 further comprising a subscriber
distribution system located within the premises of the subscriber
and connected to said cable distribution system, wherein said
adaptive filter is located at a juncture of said cable distribution
system and said subscriber distribution system.
23. The apparatus of claim 18 further comprising a D/A converter
coupled between said adaptive filter and said cable distribution
system.
24. The apparatus of claim 23 further comprising a modulator
coupled between said D/A converter and said cable distribution
system.
25. The apparatus of claim 18 wherein said central distribution
point is an optical fiber node.
26. The apparatus of claim 18 wherein said adaptive filter is a FIR
filter.
27. The apparatus of claim 18 further comprising a headend for
transmitting television signals to said central distribution point,
wherein said central distribution point is an optical fiber node
connected to said headend by an optical fiber, said adaptive filter
being located at said headend, said antenna being located at said
fiber node and transmitting antenna signals to said adaptive filter
via said optical fiber.
28. The apparatus of claim 27 wherein said adaptive filter is a FIR
filter.
29. In a cable television having a cable distribution system for
distributing television signals downstream from a central
distribution point to a plurality of subscribers, each said
subscriber having a subscriber distribution system located within
the premises of the subscriber and connected to said cable
distribution system, and for transmitting data signals from at
least one of said subscribers upstream to said central distribution
point, noise interference cancellation apparatus comprising:
an antenna local to each of said subscribers for receiving radio
frequency interference signals in the range of substantially 5-50
MHz which interfere with said upstream signals and for generating
an antenna signal;
an adaptive filter located at a juncture of said cable distribution
system and said subscriber distribution system for receiving said
antenna signal and said upstream signal for cancelling said radio
frequency interference by correlating said antenna signal and said
upstream signal, said adaptive filter comprising:
a first bandpass filter coupled to said antenna;
a first demodulator coupled to said first bandpass filter;
a first A/D converter coupled to said first demodulator;
a second bandpass filter coupled to said upstream signals;
a second demodulator coupled to said second bandpass filter;
a second A/D converter coupled to said second demodulator;
a correlator coupled to said first and second A/D converters for
reducing interference in said upstream signals;
a D/A converter coupled to said adaptive filter; and
a modulator coupled between said D/A converter and said cable
distribution system.
30. In a cable television having a cable distribution system for
distributing television signals downstream from a headend via an
optical fiber to a fiber node and from said fiber node via coaxial
cable to a plurality of subscribers, each said subscriber having a
subscriber distribution system located within the premises of the
subscriber and connected to said cable distribution system, and for
transmitting data signals from at least one of said subscribers
upstream to said central distribution point, noise interference
cancellation apparatus comprising:
an antenna located at said fiber node for receiving radio frequency
interference signals in the range of substantially 5-50 MHz which
interfere with said upstream signals and for generating an antenna
signal;
an adaptive filter located at said headend for receiving said
antenna signal via said optical fiber and said upstream signals for
cancelling said radio frequency interference by correlating said
antenna signal and said upstream signal, said adaptive filter
comprising:
a first bandpass filter coupled to said antenna;
a first demodulator coupled to said first bandpass filter;
a first A/D converter coupled to said first demodulator;
a second bandpass filter coupled to said upstream signals;
a second demodulator coupled to said second bandpass filter;
a second A/D converter coupled to said second demodulator;
a correlator coupled to said first and second demodulators for
reducing interference in said upstream signals.
31. In a cable television having a cable distribution system for
distributing television signals downstream from a headend via an
optical fiber to a fiber node and from said fiber node via coaxial
cable to a plurality of subscribers, each said subscriber having a
subscriber distribution system located within the premises of the
subscriber and connected to said cable distribution system, and for
transmitting data signals from at least one of said subscribers
upstream to said central distribution point, noise interference
cancellation apparatus comprising:
a first antenna located at said fiber node for receiving radio
frequency interference signals in the range of substantially 5-50
MHz which interfere with said upstream signals and for generating a
first antenna signal;
a first adaptive filter located at said headend for receiving said
antenna signal via said optical fiber and said upstream signals for
cancelling said radio frequency interference by correlating said
antenna signal and said upstream signal, said first adaptive filter
comprising:
a first bandpass filter coupled to said antenna;
a first demodulator coupled to said first bandpass filter;
a first A/D converter coupled to said first demodulator;
a second bandpass filter coupled to said upstream signals;
a second demodulator coupled to said second bandpass filter;
a second A/D converter coupled to said second demodulator;
a first correlator coupled to said first and second demodulators
for reducing interference in said upstream signals; and
a second antenna local to each of said subscribers for receiving
radio frequency interference signals in the range of substantially
5-50 MHz which interfere with said upstream signals and for
generating an antenna signal;
a second adaptive filter located at a juncture of said cable
distribution system and said subscriber distribution system for
receiving said antenna signal and said upstream signal for
cancelling said radio frequency interference by correlating said
antenna signal and said upstream signal, said second adaptive
filter comprising:
a third bandpass filter coupled to said antenna;
a third demodulator coupled to said third bandpass filter;
a third A/D converter coupled to said third demodulator;
a fourth bandpass filter coupled to said upstream signals;
a fourth demodulator coupled to said fourth bandpass filter;
a fourth A/D converter coupled to said fourth demodulator;
a second correlator coupled to said third and fourth A/D converters
for reducing interference in said upstream signals;
a D/A converter coupled to said second adaptive filter; and
a modulator coupled between said D/A converter and said cable
distribution system.
Description
BACKGROUND OF THE INVENTION
Modern cable television systems (CATV) transmit television
information from a headend downstream to the subscriber and receive
data signals upstream from the subscriber to the headend. In a 70
channel system, the downstream signals typically occupy the band
from 50-500 MHz. In addition, digital channels may be transmitted
from 500-750 MHz. The upstream signals utilize the unoccupied band
from approximately 5-50 MHz.
FIG. 1 illustrates a known cable distribution system 100. In the
known system, the signal is transmitted from the headend 102
through a fiber optic cable 104 to a fiber node 106. At the fiber
node the signal is converted to an electrical signal and
distributed over coaxial cable 108 which contains a plurality of
taps 110 connecting a plurality of residential units 114 to the
distribution cable 108 via a drop cable 112. In older systems, the
signal may be transmitted from the headend directly onto the
coaxial cable 108. As used herein, the term cable distribution
system refers to the distribution cable or fiber optic cable from
the headend to the residential unit, sometimes known as the
distribution loop plant. In each residential unit, there is a cable
distribution system, referred to herein as the subscriber
distribution system, which may include a portion installed by the
cable company and a portion installed by the subscriber. The
portion installed by the subscriber is typically installed by
non-skilled persons and often utilizes cable and other components
of inferior quality to that provided by the cable television
company.
The channel capacity for the upstream signals is lower than can
optimally be provided by the 45 MHz bandwidth due to interference
signals on the lines. This results in approximately 30 percent
reduction in the upstream channel capacity. It is known that these
interference signals are off-the-air radio signals. Common radio
interference sources are international shortwave stations and local
amateur radio operators. International shortwave radio has
frequency bands at 6 MHz, 7.5 MHz, 9.5 MHz and 12 MHz, for example.
There are usually tens of radio stations active at each frequency
band. Amateur radio has a group of similar frequency bands at 7
MHz, 10 MHz and 14 MHz, for example. Amateur radio signals may
employ single sideband modulation that makes the transmit power
non-stationary.
The field strength of international shortwave radio stations is
typically 10-70 dB .mu.V/m. Short-wave radio interference could
come from anywhere on the globe. An individual amateur radio
transmitter could generate a field strength of 140 dB .mu.V/m at a
distance of 10 meters. The field strength becomes weaker as the
distance increases; being reduced to 100 dB .mu.V/m at a distance
of 1000 meters.
It is believed that the interference on the upstream signal is
caused by pickup from the cable distribution system, which is
"funneled" into the system. The "funneling" is the result of the
widespread cable lines that are all connected to the fiber node and
then to the headend. The interference signals picked up by both the
drop cables and distribution cables then add on the distribution
cable. Thus, the interference at the fiber node will be greater
than interference at any other portion in the coaxial cable
distribution portion of the cable television system, and thus
reduce the channel capacity of the upstream signals. It has also
been shown that problems in the installation of the subscriber
distribution system, caused both by use of inferior components and
by the installation by non-skilled persons, results in this being a
significant source of interference signals.
Cable television systems will become increasingly interactive.
Assuming that the channel capacity is currently reduced by 30 per
cent, a 10 dB reduction in noise should result in a 45 megabit per
second (Mbps) increase in the channel capacity, thereby allowing
significantly more data to flow in the upstream direction, thus
permitting the interactive services provided by the cable system to
be expanded.
SUMMARY OF THE INVENTION
It is a general object of the present invention to reduce noise in
upstream signals on cable television systems.
It is a further object of the present invention to reduce noise in
upstream signals in hybrid fiber optics/coaxial cable television
systems.
Another object of the present invention is to reduce radio
frequency noise for upstream signals on a cable television
system.
Yet another object of the present invention is to increase the
channel capacity for upstream signals on cable television
systems.
A further object of the present invention is to provide noise
cancellation for upstream signals at the headend.
A still further object of the present invention is to provide noise
cancellation for upstream signals at the juncture of the cable
distribution system and the subscriber distribution system.
These and other objects, advantages and features are achieved by a
cable television system having noise interference cancellation
apparatus for upstream signals. A cable distribution system
distributes television signals downstream from a central
distribution point to a plurality of subscribers and transmits data
signals upstream from at least one of the subscribers to the
central distribution point. A signal controller located at a
subscriber transmits the upstream data signals to the central
distribution point. An antenna is coupled to an adaptive filter
which is also coupled to the cable distribution system between the
signal controller and the central distribution point for cancelling
radio frequency interference in the upstream signal.
Another aspect of the invention includes a method of noise
interference cancellation for upstream signals in a cable
television system. A cable distribution system distributes
downstream television signals downstream from a central
distribution point to a plurality of subscribers and transmits data
signals upstream from at least on of the subscribers to the central
distribution point. Radio frequency signals which will interfere
with the upstream signals are received via an antenna to generate
an antenna signal. Radio frequency interference signals are
cancelled in an adaptive filter coupled to the cable distribution
system.
A further aspect of the invention comprises noise interference
apparatus for use in a cable television system having a cable
distribution system for distributing television signals downstream
from a central distribution point to a plurality of subscribers and
for transmitting data signals upstream from at least one of the
subscribers to the central distribution point. An antenna receives
interference signals and generates an antenna signal. Noise
cancelling means receives the antenna signal and the upstream
signal for cancelling interference in the upstream signal by
correlating the antenna signal and the upstream signal.
A still further aspect of the invention includes noise interference
cancellation apparatus for use in a cable television having a cable
distribution system for distributing television signals downstream
from a central distribution point to a plurality of subscribers,
each subscriber having a subscriber distribution system located
within the premises of the subscriber and connected to the cable
distribution system, and for transmitting data signals from at
least one of the subscribers upstream to the central distribution
point. An antenna local to each of the subscribers receives radio
frequency interference signals in the range of substantially 5-50
MHz which interfere with the upstream signals and generates an
antenna signal. An adaptive filter located at a juncture of the
cable distribution system and the subscriber distribution system
receives the antenna signal and the upstream signals for cancelling
the radio frequency interference by correlating the antenna signal
and the upstream signal. The adaptive filter comprises a first
bandpass filter coupled to the antenna, a first demodulator coupled
to the first bandpass filter, a first A/D converter coupled to the
first demodulator, a second bandpass filter coupled to the upstream
signals, a second demodulator coupled to the second bandpass
filter, a second A/D converter coupled to the second demodulator, a
correlator coupled to the first and second A/D converters for
reducing interference in the upstream signals, a D/A converter
coupled to the adaptive filter, and a modulator coupled between the
D/A converter and the cable distribution system.
Another aspect of the invention includes noise interference
canceling apparatus for use in a cable television having a cable
distribution system for distributing television signals downstream
from a headend via an optical fiber to a fiber node and from the
fiber node via coaxial cable to a plurality of subscribers, each
subscriber having a subscriber distribution system located within
the premises of the subscriber and connected to the cable
distribution system, and for transmitting data signals from at
least one of the subscribers upstream to the central distribution
point. An antenna located at the fiber node receives radio
frequency interference signals in the range of substantially 5-50
MHz which interfere with the upstream signals and generates an
antenna signal. An adaptive filter located at the headend receives
the antenna signal via the optical fiber and the upstream signals
for cancelling the radio frequency interference by correlating the
antenna signal and the upstream signal. The adaptive filter
comprises a first bandpass filter coupled to the antenna, a first
demodulator coupled to the first bandpass filter, a first A/D
converter coupled to the first demodulator, a second bandpass
filter coupled to the upstream signals, a second demodulator
coupled to the second bandpass filter, a second A/D converter
coupled to the second demodulator, and a correlator coupled to the
first and second demodulators for reducing interference in the
upstream signals.
Yet another aspect of the invention includes noise interference
cancellation apparatus for use in a cable television having a cable
distribution system for distributing television signals downstream
from a headend via an optical fiber to a fiber node and from the
fiber node via coaxial cable to a plurality of subscribers, each
subscriber having a subscriber distribution system located within
the premises of the subscriber and connected to the cable
distribution system, and for transmitting data signals from at
least one of the subscribers upstream to the central distribution
point. A first antenna located at the fiber node receives radio
frequency interference signals in the range of substantially 5-50
MHz which interfere with the upstream signals and generates an
antenna signal. A first adaptive filter located at the headend
receives the antenna signal via the optical fiber and the upstream
signals for cancelling the radio frequency interference by
correlating the antenna signal and the upstream signal. The first
adaptive filter comprises a first bandpass filter coupled to
antenna, a first demodulator coupled to the first bandpass filter,
a first A/D converter coupled to the first demodulator, a second
bandpass filter coupled to the upstream signals, a second
demodulator coupled to the second bandpass filter, a second A/D
converter coupled to the second demodulator, a correlator coupled
to the first and second demodulators for reducing interference in
the upstream signals; and a second antenna local to each of the
subscribers receives radio frequency interference signals in the
range of substantially 5-50 MHz which interfere with the upstream
signals and generates a second antenna signal. A second adaptive
filter located at a juncture of the cable distribution system and
the subscriber distribution system receives the antenna signal and
the upstream signals for cancelling the radio frequency
interference by correlating the antenna signal and the upstream
signal. The second adaptive filter comprises a third bandpass
filter coupled to the antenna, a third demodulator coupled to the
third bandpass filter, a third A/D converter coupled to the third
demodulator, a fourth bandpass filter coupled to the upstream
signals, a fourth demodulator coupled to the fourth bandpass
filter, a fourth A/D converter coupled to the fourth demodulator, a
second correlator coupled to the third and fourth A/D converters
for reducing interference in the upstream signals, a D/A converter
coupled to the second adaptive filter, and a modulator coupled
between the D/A converter and the cable distribution system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a known cable distribution system;
FIG. 2 is a block diagram of a cable distribution system employing
the noise cancellation technique of the present invention at the
headend;
FIG. 3 is a block diagram of the circuitry at the fiber node for
the embodiment shown in FIG. 2;
FIGS. 4 is a block diagram of the circuitry at the headend for the
embodiment shown in FIG. 2;
FIG. 5 is a block diagram of an alternate embodiment of the present
invention employing noise cancellation at the juncture between the
cable distribution system and the subscriber distribution
system;
FIG. 6 is a block diagram of the circuitry for noise cancellation
technique shown in FIG. 5; and
FIG. 7 is an alternate embodiment of the circuitry shown in FIGS. 4
and 6 employing multiple antennas for cancelling multiple frequency
signals.
DETAILED DESCRIPTION
FIG. 2 illustrates one embodiment of the present invention in which
a cable distribution system 200 employs upstream interference
signal cancellation. The system 200 is similar to the system 100
shown in FIG. 1 and similar reference numerals illustrate like
elements to that shown in FIG. 1. Thus, headend 202 is coupled by
fiber optic cable 204 to fiber node 206 from which the signal is
converted to an electrical signal which is transmitted on coaxial
cable 208. Coaxial cable 208 has a plurality of taps 210 which
couple the signal on cable 208 to the drop cable 212 and then to a
residential unit 214. Residential unit 214 contains a subscriber
distribution system (not shown) which may include components
provided by the cable company and components provided by the
subscriber.
In the embodiment shown in FIG. 200, an antenna 220 is coupled to
fiber node 206. The antenna 220 collects radio frequency signals
which may interfere with the upstream data signals along the cable
distribution system and couples the signals to fiber node 206. At
fiber node 206, the signals are modulated onto the optical fiber
cable and transmitted back via the fiber optic cable 204 to headend
202. Headend 202 contains, inter alia, upstream receiver 216 for
processing upstream signals and an ingress canceler 218 for
cancelling noise on the upstream signals. The other components at
the headend 202 for transmitting the signals downstream are well
known in the art and omitted from FIG. 2. The ingress canceler 218
separates the upstream signals into those generated by antenna 220
and those generated by the subscribers of the cable system. The two
signals are then correlated in an adaptive filter within ingress
canceler 218. The adaptive filter can be similar to adaptive
filters employed in acoustic echo cancellation.
Adaptive filters are utilized in telephone systems to remove the
echo produced by hands-free telephone (speaker phones), for
example, where the output of the speaker may be picked up by via
microphone of the hands-free telephone and transmitted back to the
far end user of the telephone system. Adaptive filters for acoustic
echo cancellation are known, for example, from N. D. Degan and C.
Prati, "Acoustic noise analysis and speech enhancement techniques
for mobile radio applications," Signal Processing, No. 15, pp.
43-56, 1988, North-Holland, which is incorporated herein by
reference.
FIG. 3 illustrates the circuitry necessary at the fiber node 206 to
transit the upstream and antenna signals to the headend via optical
fiber 204. The circuitry necessary at the fiber node for
transmitting downstream signals is omitted for clarity. The antenna
220 is coupled to band pass filter 306 which filters out signals
that are outside the band of interest for cancelling the noise on
the upstream signals. The output of band pass filter 306 is then
coupled to modulator 304 which modulates the information on a
suitable carrier so that it may be separately transmitted along the
fiber optic cable 204 without interfering with the information
provided by the upstream signals or interfering with the downstream
information. The output of modulator 304 is passed to an
electrical/optical interface 302 in which the signal is converted
to an optical signal for transmission along the fiber optic cable
304. In systems in which the fiber optic cable 304 is omitted, the
modulated signal from modulator 304 could directly be transmitted
via a coaxial link to the headend 202. The upstream signals
received via coaxial cable 208 are coupled to band pass filter 310
which eliminates signals outside the pass band for upstream
signals. The output of band pass filter 310 is passed to modulator
308 which modulates the signal on a suitable carrier for
transmission along the fiber optic cable 204 to the headend. The
output of modulator 308 is likewise coupled to electrical/optical
interface 302 for transmission along the optical fiber 204.
FIG. 4 shows the circuitry necessary at the headend for receiving
the information from the fiber node 206 and for cancelling the
interference signals from the upstream data signals. In the
interest of clarity, circuitry necessary to transmit the downstream
signals is omitted. Optical fiber cable 204 is received at
optical/electrical interface 402 which separates the signal into
two electrical signals on lines 404 and 406 because each is
received on a separate carrier frequency. The signal representing
noise collected through the antenna 220 is coupled via line 404 to
band pass filter 408 which removes signals outside the band of
interest. The output of band pass filter 408 is passed through
demodulator 412 in which the signal is reduced to a base band
signal. The output of the demodulator 412 at base band is coupled
to the A/D converter 416 in which the noise signals are digitized.
The output of the A/D converter is coupled to adaptive filter 420.
The upstream data signals on line 406 are coupled to band pass
filter 410 which removes signals outside of the band allocated for
upstream data signals. The output of band pass filter 410 is
coupled to demodulator 414 which the upstream data signals are
reduced to a base band signal. The output of demodulator 414 is
passed to A/D converter 418 in which the demodulated upstream
signals are converted to digital signals. The output of A/D
converter 418 is coupled to the plus input of summing node 430 via
line 424. The output of adaptive filter 420 is coupled via line 422
to the minus input of summing node 430. The output of the summing
node 430 on line 428 is fed via line 426 back to adaptive filter
420 in order to adjust the coefficients of the filter 420, as is
well known in the art. Line 428 also couples the output of summing
node 430 to the upstream receiver 216 at the headend, as shown in
FIG. 2.
FIG. 5 illustrates a second embodiment of the present invention
generally as 500. In this embodiment use is made of the belief that
a major source of the interference in the upstream signals is
caused by the subscriber distribution system. This second
embodiment can be used with the embodiment illustrated in FIG. 2 in
order to achieve an even greater cancellation of the noise both
from sources within the residence and from sources which may be
picked up by the coaxial cable between the residence and the fiber
node or headend. In the system of FIG. 5, a residential unit 514
has a subscriber distribution system 502 coupled to a noise
canceler 506 the output of which is coupled to drop cable 512.
Cable 512 corresponds to cable 212 shown in FIG. 2. Antenna 504,
shown located within the residence, is coupled to a second input of
noise canceler 506. Although the antenna 504 is shown within the
residence, the antenna could be outside of the residence in close
proximity to the residence in order to pick up signals which are
picked up by the subscriber distribution system within the
residence. The interference canceler 506 operates on the upstream
signals using an adaptive filter to cancel the interference caused
by the radio frequency signals. The close proximity of antenna 504
to the residence will result in the signals that are received by
the antenna correlating better with interference signal picked up
by the subscriber distribution system.
FIG. 6 illustrates the circuitry within the interference canceler
506 necessary to cancel radio frequency interference from the
upstream signals. Circuitry necessary to pass the downstream signal
from the drop cable 512 to the subscriber distribution system 502
is omitted for clarity. The radio frequency signals received by
antenna 504 are coupled to band pass filter 602 in which signals
outside the band for signals interfering with the upstream signals
are eliminated. The output of band pass filter 602 is passed to
demodulator 604 in which the interfering signals are converted to
base band signals. The output of demodulator 604 is coupled to A/D
converter 606 in which the base band signals are digitized. The
output of A/D converter 606 is coupled to adaptive filter 608. The
output of the subscriber distribution system 502 is coupled to band
pass filter 610 in which signals outside the band for upstream
signals are eliminated. The output of band pass filter 610 is
coupled to demodulator 612 in which the upstream data signals are
converted to base band signals. The base band signals on the output
demodulator 612 are coupled to A/D converter 614 in which the data
signals are digitized. The output of A/D converter 614 is fed to
the plus input of summing node 614. The output of adaptive filter
608 is fed via line 616 to the minus input of summing node 614. The
output of summing node 614 is fed via line 618 to line 620 in which
the output is fed back to the adaptive filter 608 in order to
adjust the coefficients of the adaptive filter in a manner known in
the art. Line 618 also couples the output signal to D/A converter
622 in which the output of the summing node 614 is converted to an
analog signal again. The analog signal is coupled to modulator 624
in which the signals are remodulated for transmission to the
headend. The output of modulator 624 is coupled to band pass filter
626 in which signals outside the band allocated for upstream
signals are eliminated. The output of band pass filter 626 is
coupled to drop cable 512 for transmission to the cable
distribution system.
As stated in the background portion of this application, a 10 dB
reduction in noise will result in an increase in the data handling
capacity of the channel of 45 Mbps. The following is a sample
calculation of the amount of reduction in interference that can be
achieved utilizing the present invention. Let us assume that
P.sub.li =k.sub.i P.sub.ai where P.sub.li is the power at the ith
instance from the cable distribution system, P.sub.ai is the power
from the antenna at the same instance, and k.sub.i is the random
coefficient relating ingress power at the same instance, the
correlation coefficient between ingress power from the cable
distribution system and the antenna can be calculated as shown in
the N. D. Degan and C. Prati article as: ##EQU1## For the special
case of constant power at the antenna we have ##EQU2##
TABLE 1 ______________________________________ Ingress Power
Relationship Relationship Number of Occurrences
______________________________________ P.sub.li = 0.00281P.sub.ai 2
P.sub.li = 0.011P.sub.ai 3 P.sub.li = 0.022P.sub.ai 21 P.sub.li =
0.044P.sub.ai 85 P.sub.li = 0.0891P.sub.ai 41 P.sub.li =
0.1778P.sub.ai 16 P.sub.li = 0.3548P.sub.ai 4 P.sub.li =
0.708P.sub.ai 1 ______________________________________ Using the
data of Table 1, we have ##EQU3## Hence we have ##EQU4## Assuming
Gaussian properties for ingress from the antenna and from the cable
distribution system, the correlation coefficient of ingress is
related to the correlation coefficient of ingress power as ##EQU5##
which is derived by letting the instantaneous ingress power from
the antenna be P.sub.a =x.sup.2.sub.a and the instantaneous power
from the cable distribution system be P.sub.l =x.sup.2.sub.l, we
have the correlation coefficient of ingress as ##EQU6## and the
correlation coefficient of ingress power as ##EQU7## We also have
the following relationships from K. S. Shamugan and A. M. Breipohl,
Random Signals, John Wiley and Sons, 1988, which are incorporated
herein by reference: ##EQU8## With these relationships, we can
further express R.sub.PlPa as ##EQU9##
If all the correlated ingress components can be removed the ingress
cancellation can reduce the noise level by 20.times.log.sub.10
(1-0.7304)=11.3 dB. The cancellation level could be higher if the
antenna is moved closer to the cable distribution system instead of
being placed at the headend. On the other hand the relative
shielding effectiveness spread could be smaller if all P.sub.ai 's
are really equal.
Performance of the ingress cancellation may be limited by the
amount of non-correlated noise existing at either the antenna site
or in the cable distribution system. The performance might also be
affected if the upstream signal is somehow leaked into the antenna.
Assuming that the transmission paths for the ingress through the
antenna is H.sub.1 (z) and through the cable distribution system is
H.sub.2 (z), the optimal Wiener solution for the adaptive filter
known from B. Widrow, J. R. Glover, Jr., J. M. McCool, J. Kaunitz,
C. S. Williams, R. H. Hearn, J. R. Zeidler, E. Dong, Jr., and R. C.
Doodlin, "Adaptive noise cancelling: principles and applications,"
Proc. IEEE, vol. 63, No. 12, pp. 1692-1716, December 1975, which is
incorporated herein by reference, is ##EQU10## where H(z)=H.sub.1
(z)/H.sub.2 (z), S.sub.nn (z) is the power spectrum of the ingress,
S.sub.mm (z) is the power spectrum of the antenna site additive
noise. For small S.sub.mm (z) we have W(z).apprxeq.1/H(z). If the
antenna somehow also collects the upstream signal, the Wiener
solution of the adaptive filter becomes ##EQU11## where S.sub.ss
(z) is the power spectrum of the upstream signal and F(z) is the
transmission path to the antenna.
Other factors include the magnitude and the variation rate of the
transmission path difference between the antenna and the loop
plant. The delay should be short enough to be covered by an
adaptive filter with a reasonable number of filter coefficients
operating at the sampling which is about twice of the bandwidth of
the transmission system. The delay variation should be slow enough
to be tracked by the adaptive mechanism.
Where competing ingress sources poses a problem for the single
adaptive filter approach, multiple-reference noise cancellation can
be applied to obtain a better results. As shown in FIG. 7 n
adaptive filters with corresponding antenna sites are used for
m(<n) interference sources. The system could be implemented at
the fiber node if the transmission of ingress noise collected from
many antenna sites through the fiber link is a problem. This
technique could also be applied at the residential unit canceler
shown in FIG. 5. The antenna sites should be diversified enough
such that associated ingress strength distributions are
distinguishable.
The matrix Weiner solution for the adaptive filter bank known from
the B. Widrow et. al article is
where [S.sub.xx (z)] is the ingress spectral matrix, F(z) is the
ingress to antenna transfer function matrix, and {G(z)} is the
ingress to the cable distribution system transfer function
vector.
By the radio frequency allocation rule, different radio stations
should have different carrier frequency hence the interference
between different stations could be avoided. However, the carrier
frequency overlapping could occur in the boundary between adjacent
coverage arena. This could especially happen for short wave radio
band because signals have traveled around the globe. The most
likely competing situation is expected to involve no more than two
ingress sources. Therefore a maximum number of three adaptive
filters and associated antenna sites will be sufficient.
FIG. 7 illustrates a system employing multiple antennas and
multiple adaptive filters. The first antenna 702 is coupled through
a band pass filter 710 which eliminates frequencies outside the
desired band. The output of band pass filter 710 is coupled to
demodulator 718 in which the signal converted to the base band
frequencies. The base band frequencies from the output of the
demodulator coupled through A/D converter 726 in which they are
digitized. Digital signals are then input to the first adaptive
filter 734. Similarly, antenna 704 is coupled to band pass filter
712 and demodulator 720, the output of which is digitized in A/D
converter 728 and that is an input to the second adaptive filter
736. The nth antenna 706 is coupled to band pass filter 714 the
output of which is demodulated in demodulator 722 and digitized in
A/D converter 730, the output of which is fed as an input to the
nth adaptive filter 738. The outputs of the adaptive filters are
fed to summing point 740. The upstream signals are coupled via line
708 to band pass filter 716 in which frequencies outside the band
for the upstream signals are eliminated. The output of band pass
filter 716 is coupled through demodulator 724 which converts the
signals to base band signals. Base band signals from demodulator
724 are coupled to A/D converter 732 in which they are digitized.
The digital signals output from A/D converter 732 are coupled via
line 739 to the plus input of summing point 742. The output of
summing point 740 is coupled via line 741 to the minus input of
summing point 742. The output of summing point 742 is coupled via
line 743 to D/A converter 744. The output is also coupled via line
745 to the adaptive filters for adjusting the coefficients thereof
in a known manner. The output of D/A converter 744 is fed to
modulator 746 in which the signals are remodulated for transmission
to the headend. The output of modulator 746 is coupled to
electrical/optical interface 748 in which the signals are converted
to an optical signal which is then coupled to an optical fiber 750.
If the upstream signals are to be placed on a coaxial cable, such
as in the example shown in FIG. 5, then the electrical/optical
interface 748 is eliminated and the signal is placed directly on to
a coaxial cable 750.
While a particular embodiment of the present invention has been
disclosed herein, certain changes and modifications will readily
occur to those skilled in the art. All such changes and
modifications can be made without departing from the invention as
defined by the appended claims.
* * * * *